Preparation of cyano-containing perfluoropolymers having iodine curesites

ABSTRACT

In the preparation of perfluoropolymers by the interpolymerization of tetrofluoroethylene, perfluro (alkyl vinyl) ether and cyano-cure site monomer, the improvement of incorporating iodo moieties by the reaction of iodo-compounds of the formula RIn, in which R is a hydrocarbon or halocarbon radical of 1 to 8 carbon atoms and n is 1 or 2.

BACKGROUND OF THE INVENTION

Perfluoroelastomers have long been used in a variety of applicationsthat require excellent resistance to high temperature and chemicalattack. One particularly outstanding fluoropolymer that has been used inelastomeric applications is that prepared from tetrafluoroethylene (TFE)and perfluoro (alkyl vinyl) ether, and particularly perfluro (methylvinyl) ether (PMVE) To facilitate the crosslinking in these copolymersthat is essential to good elastomeric properties, a small percentage oftermonomer is generally incorporated, such as the cyano-containingfluoroolefins described in Breazeale, U.S. Pat. No. 4,281,092.

While these cyano-perfluoroelastomers have exhibited outstanding thermaland oxidative stability, continuing effort has been directed to furthermodification of these polymers to broaden the areas in which they can beused. Particular attention has been paid to the rate and thoroughness ofthe curing reaction that has been used with perfluoroelastomers.Specifically, the perfluoropolymers previously made from TFE, PMVE and acure site monomer typically exhibit a high concentration of ionic endgroups, such as carboxylic or sulfonic acid moieties, which areintroducted from the initiating system. Conventional chain transferagents can be used to lower the concentration of the ionic end groupsand also lower the polymer viscosity. However, the physical propertiesof these polymers are generally depreciated with such chain transferagents.

SUMMARY OF THE INVENTION

The instant invention relates to improved perfluoropolymer compositionswhich, when compounded and cured as elastomers, exhibit an excellentcombination of curing and mold release characteristics.

Specifically, the present invention provides, in a process for thepreparation of perfluoropolymer by the random copolymerization oftetrafluoroethylene, perfluoro (alkyl vinyl) ether and fluorinatedcuresite monomer containing nitrile groups, the improvement wherein thecopolymerization reactants further comprise compounds of the formulaRI_(n), in which R is a hydrocarbon or halocarbon radical of 1 to 8carbon atoms and n is 1 or 2.

DETAILED DESCRIPTION OF THE INVENTION

The basic components of the present perfluoropolymers, thetetrafluoroethylene and the perfluoro (alkyl vinyl) ether, and theirpolymerization, are desribed in Harris et al, U.S. Pat. No. 3,132,123,which is hereby incorporated by reference. Of the perfluoro (alkylvinyl) ethers, perfluoro (methyl vinyl) ether (PMVE) has been found tobe particularly satisfactory. Other perfluoro (alkyl vinyl) ethers whichcan be used in the present invention include perfluoro (alkoxy alkylvinyl) ethers, such as those described in Fritz et al., U.S. Pat. No.3,291,843, also hereby incorporated by reference. Of those perfluoro(alkoxy alkyl vinyl) ethers, perfluoro (5-methyl-3,6-dioxa-1-nonene) hasbeen found to be particularly satisfactory.

A wide variety of cyano- cure sites can be incorporated into thesepolymers, including those described in Breazeale, U.S. Pat. No.4,281,092, hereby incorporated by reference. The polymers resulting formthis invention preferably contain such curesites randomly distributedalong the backbone of the perfluoropolymer.

A central aspect of the present invention is that the perfluoropolymercomprise iodine moieties. The iodo moieties can be incorporated byconducting the radical copolymerization of the basic monomers notedabove in the presence of an iodine-containing compound represented bythe formula RI_(x), where R is a hydrocarbon or saturated fluorocarbonor chlorofluorocarbon residue having 1 to 8 carbon atoms and x is aninteger of 1 or 2, corresponding to the valence of the residue R.

In the course of the radically initiated copolymerization, the iodocompound acts as a chain transfer agent, resulting in a telomerizationpolymerization process in which a labile, iodine-containing chain end isformed, and the haloalkyl residue of the iodo compound is attached tothe other end of the polymer chain. If the iodo compound has two iodogroups, the fluoropolymer chain may therefore have iodine groups at eachend, and the telomerization polymerization process will occur at eachend of the polymer chain.

Iodo-compounds which can be used in the preparation of theperfluoropolymers according to the present invention include, forexample, those based on hydrocarbon residue such as methylene iodide,1,4-diiodo butane, and butyl iodide, and those based on saturatedfluorocarbon or chlorofluorocarbon residue such as monoiodoperfluoromethane, diiodo methane, monoiodo perfluoroethane, monoiodoperfluoro propane, monoiodopropane, 1,3-diiodoperfluoro-n-propane,1,4-diiodo-n-butane,1,4-diiodoperfluoro-n-butane,1,3-diiodo-2-chloroperfluoro-n-propane and1,5-diiodo-2,4-dichloro-perfluoro-n-pentane. Other iodo-compounds whichcan be used include those described in U.S. Pat. No. 4,243,770, herebyincorporated by reference.

The amount of iodo-compound used should be high enough to give extensivechain transfer and result in incorporation of at least about 0.05 weight% of iodine in the perfluoropolymer. High chain transfer efficiency bythe alkyl iodide results in a perfluoropolymer with lower compoundviscosity and a relatively narrow molecular weight distribution with atypical value of Mw/Mn of about 2-3, for desirable rheology andprocessing characteristics.

In general, the concentration of iodine in the polymer should be aboutfrom 0.05 to 1.0 weight %, and preferably 0.1-0.5 weight %, based on theperfluoropolymer composition. The concentration in the fluoropolymerwill depend upon the concentration of alkyl iodides in thepolymerization medium and upon polymerization conditions, which willeffect the chain transfer efficiency. The upper limit on iodine contentcorresponds approximately to the practical lower limit on polymerviscosity, since higher concentrations of iodine gives polymers withlower molecular weight and viscosity. The iodine concentration in theperfluoropolymer can be determined by conventional analyticaltechniques, such as elemental analysis.

The lower limit of iodine incorporation is approximately that at which asignificant effect on the cure rate and vulcanizate properties is foundwhen cured by peroxides. The upper limit on iodine content correspondsapproximately to the practical lower limit on polymer viscosity, sincehigher concentrations of iodine gives polymers with lower molecularweight and viscosity. The upper limit on iodine content also relates tothe desired highest state of cure, insofar as it relates to theefficiency of formation of chains terminated with iodo groups.

The components of the perfluoropolymers are present in the amountsgenerally used in the preparation of copolymers of tetrafluoroethylene,perfluoro (alkyl vinyl) ether and cure site monomer, as described, forexample, in the aforementioned U.S. Pat. No. 4,281,092. Concentrationsof about from 0.1 to 5.0 weight % of the cyano- moiety have been foundto provide particularly good processing characteristics.

According to the present invention, the perfluoropolymers are made byfree radical emulsion polymerization in a continuous stirred tankreactor, as described, for example, in the aforementioned U.S. Pat. No.4,281,092, and under the general reaction conditions used in the pastfor the preparation of perfluoropolymers. Specifically, polymerizationtemperatures can be in the range of about from 40 to 130° C., andpreferably about from 70° to 115° C., at pressures of about from 2 to 8MPa and residence time 10 to 240 minutes. Free radical generation iseffected using a water-soluble initiator such as ammonium persulfate,either by thermal decomposition or by reaction with a reducing agentsuch as sodium sulfite. The alkyl iodides can be fed into the reactordirectly or as a solution. Initiator levels are set low enough so thatiodine end groups predominate over those from initiator fragments. Thisleads to the desired low polymer viscosity and high solubility. Thepolymer dispersion is stabilized with an inert surface-active agent suchas ammonium perfluorooctanoate, usually with addition of a base such assodium hydroxide or a buffer such as disodium phosphate to control pH inthe range 3 to 7.

After polymerization, unreacted monomer is removed from the reactoreffluent latex by vaporization at reduced pressure. Polymer can berecovered from latex by coagulation, e.g., by reducing pH to about 3 byacid addition and adding a salt solution such as calcium nitrate,magnesium sulfate, or potassium aluminum sulfate in water, followed byseparation of serum from polymer, washing with water, and drying of thewet polymer. The iodine concentration in the perfluoropolymer can bedetermined by conventional analytical techniques, such as elementalanalysis.

Perfluoropolymers made according to the present invention are typicallycompounded with one or more of the additives known to be useful inperfluoropolymer compositions, such as pigments, fillers, pore-formingagents and plasticizers. It is particularly advantageous to add carbonblack to the fluoroelastomer to increase its modulus. Usually amounts offrom 5-50 parts per hundred parts of fluoroelastomer are used, with theparticular amount determined from the particle size of the carbon blackand the desired hardness and modulus of the cured composition.

The compounds are generally cured by a free radical process. A curablecomposition comprises polymer and a peroxide to generate free radicalsat curing temperatures. A dialkyl peroxide which decomposes at atemperature above 50° C. is especially preferred when the composition isto be processed at elevated temperatures before it is cured. Adi-tertiarybutyl peroxide having a tertiary carbon atom attached toperoxy oxygen may be particularly beneficial in many cases. Among themost useful peroxides of this type are2,5-dimethyl-2,5-di(tertiarybutylperoxy) hexyne-3 and2,5-dimethyl-2,5-di(tertiarybutylperoxy) hexane. Other peroxides can beselected from such compounds as dicumyl peroxide, dibenzoyl peroxide,tertiarybutyl perbenzoate, and di[1,3-dimethyl-3-(t-butylperoxy)butyl]carbonate, and the like.

The resulting perfluoroelastomers exhibit particularly good processingcharacteristics, including curing and mold release properties.Perfluoroelastomers of TFE, PMVE and cure site monomer have typicallybeen difficult to process in milling, extrusion and molding operations,because of their high bulk viscosities. While the superior performanceof the present compositions is not fully understood, it is believed thatthe selection of iodo compounds, which function as chain transferagents, results in iodo end groups for the polymer chains, and these endgroups are reactive to crosslinking agents. When used in conjunctionwith the known cyano- cure sites, the iodides permit the curing ofrelatively low molecular weight polymers with peroxides to give partswith very good properties. The polymers can be cured with greater easeand with high yields. In addition, complicated parts which could not bemade with previously available perfluoroelastomers can be fabricatedwith the present perfluoroelastomers using transfer and injectionmolding techniques.

The perfluoropolymers made according to the present invention react withperoxide curing agents and crosslinking coagents to give unique polymernetworks in which crosslinks are formed both at random points along thepolymer chain and at the ends of the chains. Such polymers haveexcellent strength and compression set properties, as well as goodprocessing characteristics.

The perfluoropolymers per se, without fillers or curing, can also beused in a wide variety of applications. In addition to the usualapplications for which perfluoroelastomers have previously been found toprovide particular benefits, the present compositions have been found tobe particularly useful as antireflective coatings for pellicles, forexample, in the protection of printed circuits. Specifically, a coatingof perfluoroelastomers of the present invention of about from 0.2 to 1.0microns, on a substrate of nitrocellulose, provides a pellicle materialhaving an outstanding combination of adhesion to both the nitrocellulosefilm and to the pellicle frame as well as excellent opticaltransmittance.

The present invention is further illustrated by the following specificexamples.

EXAMPLES 1-5 AND CONTROL EXAMPLE A

In Example 2, perfluoroelastomer was prepared from TFE, PMVE, andperfluoro-(8-cyano-5-methyl-3,6-dioxa-1-octene) (8CNVE), withperfluoroalkyl iodides (RfI or IRfI), which provided iodo moieties onthe ends of some of the polymer chains. This polymer had 8CNVE curesites randomly dispersed in the backbone.

The polymers were prepared in a 1 liter mechanically agitated,waterjacketed, stainless-steel autoclave operated continuously at 90° C.and 4800 KPa into which was pumped, at a rate of 250 ml/hr an aqueouspolymerization medium/initiator solution comprising of 8 liters ofwater, 32 g. ammonium persulfate, 260 g. of disodium hydrogen phosphateheptahydrate, and 280 g. of ammonium perfluoroctanoate ("Fluorad"FC-143, 3M Co.). At the same time, a separate solution of 8CNVE in F-113"Freon" at the rate of 5.6 ml/hr (the solution contained 3.0 g of 8CNVE,the rest being F-113) and another separate solution of 3.4 ml/hr of1,4-diiodoperfluorobutane in F-113 (0.67 g. the rest being F-113) werebeing pumped in. A gaseous stream of tetrafluoroethylene (70 g/hr) andperfluoro(methyl vinyl) ether (85 g/hr) was also fed simultaneously tothe autoclave at a consant rate by means of a diaphragm compressor.

Polymer latex was removed by means of a let-down valve and unreactedmonomers were vented. The latex from 14 hrs operations was added withstirring to a preheated (95° C.) coagulating solution consisting of 1840g MgSO₄.7H₂ O in 40 liters of water. The coagulated crumb was filteredoff, washed repeatedly with water and dried by heating in an 80° C. ovenfor 48 hrs in the presence of air. The dried polymer weighed 1825 gs.and had the composition, as weight % of the polymer, oftetrafluoroethylene 55.6, perfluoro(methyl vinyl) ether 42.6, 8CNVE 1.4and iodine 0.32. The inherent viscosity of the polymer was determined at30° C. using 0.2 g. of polymer per deciliter of a solvent mixturecomprising (by volume) 60 parts 2,2,3-trichloro-heptafluorobutane, 40parts perfluoro(butyltetrahydrofuran) and 3 parts diethylene glycoldimethyl ether. The Mooney viscosity of the polymer was measured after10 minutes as 42 at 100° C. and 12 at 121° C. The resulting polymer wasmixed on a 2-roll rubber mill with (phr) 15 MT black, 3 triallylisocyanurate, and 5 Luperco 101XL (Lubrizol Co) peroxide, and shaped andcured at 177° C. and postcured at 232° C. unrestrained under nitrogenfor 26 hrs. The results are shown in Table 1.

In the remaining Examples, the above general procedure was repeated. InControl Example A, in which no iodine was incorporated, the cure ratewas slowest, and the vulcanizate properties were inferior to those ofExamples 1-5. The operating conditions for these Examples are summarizedbelow.

    ______________________________________                                        Example   A       1       2     3     4     5                                 ______________________________________                                        TFE g/hr  70      70      70    70    70    70                                PMVE g/hr 74      85      85    85    85    85                                8CNVE g/hr                                                                              4.8     0       3.0   1.6   2.0   2.4                               I(CF.sub.2)4I g/hr                                                                      0       1.1     0.67  0.64  0.64  0.64                              APS g/hr  1.0     0.4     0.4   0.4   0.4   0.4                               Phosphate g/hr                                                                          3.5     3.2     3.2   3.2   3.2   3.2                               FC-143 g/hr                                                                             4.5     4.0     4.0   4.0   4.0   4.0                               Temp. °C.                                                                        80      90      90    90    90    90                                ______________________________________                                    

The resulting polymers were compounded, cured and tested tested asbefore, and the results are shown in Table 1.

                                      TABLE 1                                     __________________________________________________________________________    IODOMODIFIED (ICF.sub.2).sub.4 I) 8CNVE CONTAINING TFE/PMVE POLYMERS          Examples     Control A                                                                           1    2    3    4    5                                      __________________________________________________________________________    Raw Polymer Description                                                       TFE wt %     54.9  56.3 55.6 56.1 55.2 55.5                                   PMVE wt %    42.7  43.3 42.6 42.9 43.5 43                                     8CNVE wt %   2.4   --   1.4  0.8  1.1  1.2                                    8CNVE mol/kg 0.062 --   0.036                                                                              0.021                                                                              0.028                                                                              0.031                                  I wt %       --    0.40 0.32 0.24 0.24 0.29                                   I mol/kg     --    0.033                                                                              0.025                                                                              0.019                                                                              0.019                                                                              0.023                                  Inh. Viscosity                                                                             0.46  0.30 0.32 0.35 0.33 0.34                                   Mooney at 100° C.                                                                   --    35   42   65   26   52                                     121° C.                                                                             56    10   12   27   13   16                                     Cured Properties                                                              ODR 177° C.                                                            Minimum - N.m                                                                              0.65  0.2  0.15 0.35 0.2  0.25                                   mH - N.m     2.6   4.0  4.3  3.7  3.9  4.0                                    ts2 - Mins   1.5   0.9  0.9  0.9  1.1  1.1                                    TC90 - Mins  7.5   1.3  2.5  2    3.2  2.8                                    Tensiles                                                                      M100         510   1247 1552 1603 1273 1300                                   Tb           1386  1953 1834 1838 2092 1975                                   Eb           200   149  114  157  162  141                                    Hardness shore A                                                                           71    80   80   79   76   77                                     Comp, Set Resistance                                                          200° C./70 hrs                                                         Pellets      35    41   41   55   39   38                                     O-ring       35    55   46   58   39   32                                     __________________________________________________________________________     1. The compounds contained 100 parts rubber, 15 parts MT black, 3 parts o     triallylisocyanurate 5 parts Luperco 101XL peroxide.                          2. The samples were presscured at 177° C./15 mins and postcured at     232° C. for 26 hours.                                             

EXAMPLES 6-8 AND CONTROL EXAMPLE B

In Examples 6-8, TFE/PMVE/8CNVE perfluoroelastomers were prepared withhydrocarbon alkyl iodides (RI or IRI), which provided iodo moieties onthe ends of some of the polymer chains. The 8CNVE cure sites in thesepolymers were randomly dispersed in the backbone.

The polymerization conditions were identical to those of Examples 1-5,except as reported below:

    ______________________________________                                        Example       Control  6        7    8                                        ______________________________________                                        TFE g/hr      70       60       60   60                                       PMVE g/hr     74       75       75   75                                       8CNVE g/hr    4.8      3.2      3.2  3.2                                      ICH.sub.2 I g/hr                                                                            0        0.12     0.10 0.83                                     Phosphate g/hr                                                                              3.5      4.0      4.0  4.0                                      FC-143        4.5      3.6      3.6  3.6                                      Temp. °C.                                                                            80       90       90   90                                       ______________________________________                                    

The methylene iodide was dissolved in t-butanol as a 10% solutionbecause it was not soluble in F113. The compounding and curing wereidentical to the conditions used in Examples 1-5, and the results areshown in Table 2. The iodo-containing polymers of the present inventionexhibit faster cure rates that the Control Example and improvedvulcanizate properties.

                  TABLE 2                                                         ______________________________________                                        IODOMODIFIED (CH.sub.2 I.sub.2) 8CNVE                                         CONTAINING TFE/PMVE POLYMERS                                                  Examples    Control B 6        7      8                                       ______________________________________                                        Raw Polymer                                                                   Description                                                                   TFE wt %    54.9      58.3     56.7   57.6                                    PMVE wt %   42.7      40       41.5   40.5                                    8CNVE wt %  2.6       1.6      1.7    1.8                                     8CNVE mol/kg                                                                              0.062     0.041    0.044  0.046                                   I wt %      --        0.08     0.08   0.10                                    I mol/kg    --        0.006    0.006  0.008                                   Inh. Viscosity                                                                            0.46      0.45     0.45   0.42                                    Mooney at 100° C.                                                                  --        110      104    116                                     121° C.                                                                            56        57       52     50                                      Cured Properties                                                              ODR 177° C.                                                            Minimum - N.m                                                                             0.65      0.95     0.80   0.90                                    mH - N.m    2.60      3.80     3.90   3.95                                    ts2 - Mins  1.5       1.0      1.0    1.1                                     TC90 - Mins 7.5       3.5      3.1    2.6                                     Tensiles                                                                      M100        510       1213     1191   1196                                    Tb          1386      2181     2215   2104                                    Eb          200       156      167    154                                     Hardness shore A                                                                          71        75       74     76                                      Comp, Set Resis-                                                              tance 200° C./70 hrs                                                   Pellets     35        43       41     44                                      O-ring      35        28       27     35                                      ______________________________________                                         1. The compounds contained 100 parts rubber, 15 parts MT black, 3 parts o     triallylisocyanurate 5 parts Luperco 101XL peroxide.                          2. The samples were press cured at 177° C./15 mins and postcured a     288° C. for 46 hours.                                             

I claim:
 1. In a process for the preparation of perfluoropolymer by therandom copolymerization of tetrafluoroethylene, perfluoro (alkyl vinyl)ether and fluorinated curesite monomer containing nitrile groups, theimprovement wherein the copolymerization reactants further compriseiodo-compounds of the formula RI_(n), in which R is a hydrocarbon orhalocarbon radical of 1 to 8 carbon atoms and n is 1 or
 2. 2. A processof Claim 1 wherein the R group in the iodo-compound is a hydrocarbonradical.
 3. A process of Claim 1 wherein the R group in theiodo-reactant is a halocarbon radical.
 4. A process of Claim 1 whereinthe iodo-compound is a di-iodo compound.
 5. A process of Claim 4 whereinthe iodo-compound consists essentially of diiodoperfluorobutane.
 6. Aprocess of Claim 4 wherein the iodo-compound consists essentially ofdiiodoperfluorohexane.
 7. A process of Claim 2 wherein the iodo-compoundconsists essentially of methylene iodide.
 8. A process of Claim 1wherein the perfluoro (alkyl vinyl) ether consists essentially ofperfluoro (methyl vinyl) ether.
 9. A process of Claim 1 wherein theperfluoro (alkyl vinyl) ether is a perfluoro (alkoxy alkyl vinyl) ether.10. A process of Claim 9 wherein the perfluoro (alkoxy alkyl vinyl)ether consists essentially of perfluoro (5-methyl-3,6-dioxa-1-nonene).11. A process of Claim 1 wherein the nitrile curesites comprise at leastabout 0.1% by weight of the perfluoropolymer, and are randomlydistributed along the backbone of the perfluoropolymer.
 12. A process ofClaim 1 wherein the interpolymerization is carried out in a continuousstirred tank reactor.